1. Technical Field
The purpose of this invention is an iterative process for TDMA (Time Division Multiple Access) radiomobile communications.
It may be applied to the European GSM radiomobile system [1] and its development to GSM++ or to the American D-AMPS system.
2. State of Prior Art
TDMA systems can be used to distribute several users communicating through the same radioelectric channel, in time [1, 3]. Each user is separated from the others by periodically assigning a timeslot to him in which he can transmit, and another timeslot in which he can receive. Protective intervals are allowed at each end of each timeslot in order to guard against imperfections in time synchronization between transmitters and interference between consecutive timeslots resulting from the plurality of pathways.
FIG. 1 attached thus shows timeslots TA1, TA2, . . . , TAN allocated to N users with protective intervals IG.
The radioelectric signal transmitted during a timeslot is obtained by transposition of the equivalent base band signal into frequency. The base band signal is the result of filtering the data stream to be transmitted through a “transmission” filter. Normally, the transmitted data stream is composed of two sequences of data symbols separated in time by a sequence of symbols known to the receiver, called the reference sequence. The two sequences of data symbols may possibly originate from coding and interlacing of information to be transmitted.
The reference sequence frequently has good time correlation properties, in order to enable a precise estimate of the channel at the receiver. For example, the reference sequences used in the GSM system, called CAZAC (Constant Amplitude—Zero AutoCorrelation) sequences [1, 4, 5] are sequences with components taken from the bipolar alphabet {−1, 1} and possessing a null circular autocorrelation function everywhere except at the origin.
The radiomobile channel used during a communication between a transmitter and a receiver is usually of the multipath type with fast fading called RAYLEIGH fading. The existence of several paths is due to the fact that the radioelectric wave is propagated along several paths between the transmission location and the reception location.
The received signal is then the sum of several more or less delayed replicas, more or less modified in phase and in amplitude. In order to obtain a reliable reproduction of the information carried by these replicas, the receiver applies filtering matched to the transmission filter and to the channel and it combines energy contributions from all transmitted signal replicas in an optimum manner. The output signal from the matched filter is sampled at the rate of the symbols and it is whitened using a discrete filter called a whitener.
Samples at the output from the whitener filter supply a filtered and noisy version of the transmitted data stream. The discrete filter associated with this filtered version, also called the discrete channel, has a finite pulse response that varies from one sample to the other. It characterizes the production of the multipath channel during the corresponding reception timeslot, in an indirect manner.
Samples at the output from the whitener filter corresponding to the reference sequence are used to estimate the discrete channel [1, 4, 5]. This estimate of the discrete channel is used to equalize the remaining samples, so that the two transmitted data sequences can be detected if there are any. The equalizer usually used is known as the VITERBI equalizer with flexible decisions [2]. As its name suggests, this equalizer uses an application of the Soft-Output Viterbi Algorithm (SOVA) [2] to produce soft decisions on all transmitted data symbols. These soft outputs are possibly de-interlaced and decoded to detect the information transmitted.
FIG. 2 attached shows these transmission/reception operations. The transmitter E comprises a data source 10, an encoder/interlacer 12 outputting symbols ak, and a modulator 14. The multipath channel is symbolized by a block 20. The receiver R comprises a demodulator 30 (matched filter/whitener filter) outputting samples Rk, a discrete channel estimator 32, an equalizer 34, a de-interlacer/decoder 36 and finally an addressee 38.
The discrete channel seen at the output from the whitener filter may vary significantly from one timeslot to the next. This variation is due mainly to the change in propagation conditions between the transmitter and the receiver and to the frequency stability at the receiver.
Propagation conditions have a direct influence on the observed multipath channel. They change due to a modification in the environment or a displacement of the transmitter and/or the receiver. They create a time variation in the discrete channel, both between successive timeslots and within the same timeslot.
The variation of the discrete channel between two successive timeslots allocated to the same user is particularly large when the time interval between these timeslots is large. This variation is accentuated, even at a single timeslot, due to an increase in the carrier frequency or the speed of the transmitter and/or the receiver.
In practice, the variation of the discrete channel between two timeslots is sufficiently large to prevent any modulated estimate of the channel. The estimate of the channel in a timeslot must then be based solely on the samples of the corresponding reference sequence.
The use of this reference sequence provides an unbiased estimate of the discrete channel at the mid-point of the timeslot. Like the GSM system, an invariable channel is usually assumed within a received timeslot. In this precise case, the estimate obtained at the mid-point of the frame can be used without degradation for equalization of the rest of the frame. But high speed movements of some terminals and the sustained demand for higher rate services operating at increasingly high radioelectric frequencies, make this assumption less and less justified. The discrete channel may be affected by significant variations between the beginning and the end of a given timeslot. The difference between this real discrete channel and its estimate becomes increasingly large as the distance from the reference sequence increases. This may cause a large and irreversible deterioration in the reception quality and/or performances of the TDMA system.
One solution [13] has already been proposed to solve this problem of fast variation of the channel in the case of radioelectric channels related to satellite radiocommunications. However, this solution only deals with the case of one channel with a single path and assumes that reference or data symbols have a common transmitted energy. Therefore, it cannot be applied to multipath channels encountered in land radiomobile communications.
Furthermore, like the EDGE extension in the GSM system, high rates may be obtained with unchanged frequency band by the use of modulations using an increasing number of states. The system then becomes very sensitive to the channel estimating quality, and severe deterioration may occur even at very low displacement speeds if the power and/or length of the reference sequence are not increased.
The purpose of this invention is to overcome these disadvantages.